Such belt-drive transmissions are generally known. Continuously variable belt-drive transmissions usually comprise a variator for ratio adjustment which has a first cone pulley pair upon an input shaft and a second cone pulley pair upon an output shaft, and a belt-drive means running between the cone pulley pairs. Each cone pulley pair consists of one first pulley (a fixed pulley) stationary in an axial direction and one second cone pulley (an adjusting pulley) displaceable in an axial direction. The input shaft of the variator is usually designated as the primary shaft and the first cone pulley pair accordingly as the primary pulley pair. The output shaft of the variator is, as a rule, similarly designated as the secondary shaft and the second cone pulley pair as the secondary pulley pair. The primary pulleys and secondary pulleys and thus the ratio are axially adjusted by a pressure medium. The pressure medium is usually conveyed via ducts onto one or more pressure chambers of the primary or secondary pulley, and a pressure-medium pump making available the oil pressure needed.
The pressure supply of the whole transmission is mostly made possible via a pressure-medium pump directly coupled with the input rotational speed. The volume of oil conveyed depends therefore directly on the input rotational speed. When now the input rotational speed drops down to zero, the transmission is no longer supplied with oil and a so-called pressureless state appears. In this state, the maximum ratio is usually adjusted. At the same time, the primary pulleys stay insofar as possible, apart from each other and the belt-drive means rests on the primary side upon the smallest possible friction radius. The friction radius of the belt-drive means upon the output shaft, which radius is preset by the secondary pulleys, is thus at a maximum. When the motor vehicle now has to be towed in this condition, serious problems can arise:
The drive wheels of the vehicle guide torque into the transmission which is in the pressureless state. The torque is transmitted to the drive wheel via the output shaft and the belt-drive means. In the pressureless state, since a minimum friction radius adjusts on the input side and a maximum friction radius accordingly on the output side, the input shaft has to accelerate very intensively. In the pressureless state, since the compression force of the cone pulleys on the primary side is minimal, a skidding of the belt-drive means upon the primary pulleys can occur under the intensive acceleration whereby the primary pulleys are seriously damaged.
Due to the skidding, it can also occur that one clutch connected with the input shaft is intensively accelerated even when the vehicle motor is idle. When a certain limit rotational speed is exceeded, an undesired engaging of the clutch can occur in the clutch arrangement due to the rotational pressure then appearing. Due to the trailing torque then prevailing, an overheating of the clutch and the damage thereof can result.